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By Jeff Wing
July 17, 2016
Teflon. Super Glue. Silly Putty. The Microwave oven. Brilliant if accidental discoveries define much of our modern world. To that odd list you can add Concrete Air Entrainment––modern construction’s best friend. As any concrete enthusiast knows, Air Entrainment is the process by which extremely tiny, uniformly distributed bubbles are deliberately introduced into liquid concrete. Why is this done, exactly?
Fluctuations in temperature are the natural enemy of concrete magnates everywhere, particularly in colder climes. In places where air temperatures can seasonally swing from hot to cold, the “freeze-thaw” phenomenon wreaks havoc on concrete. How can a little weather wreck a slab of concrete?
You wouldn’t guess this in the moments after dropping a heavy chunk of concrete onto your bare toe, but in fact, concrete is extremely porous, and it drinks in moisture like an inefficient but determined sponge. In time, all that moisture becomes trapped in the natural fissures formed inside concrete as it hardens. When the weather outside gets frosty, so does the trapped water. Since water increases in volume when it turns to ice, the trapped water expands within the concrete until “spalling” occurs, causing a sidewalk to look like someone has taken a sledgehammer to it. The moisture trapped in the concrete swells until it has no choice but to break out through an unsightly crack of its own creation.
So what's with this Air Entrainment business? The entrainment process scatters millions of micro-bubbles uniformly throughout the concrete matrix via the introduction of surfactants that produce the effect. These ridiculously tiny bubbles are 10 to 500 microns in diameter (a human hair is around 50 microns in diameter), and when properly deployed, comprise about 6% of the host concrete's volume.
The net effect of all those fine bubbles in the concrete? They act as a sort of internal shock absorber. When moisture inevitably does get inside the concrete and attempts to swell into the surrounding space, the concrete’s inner structure is able to flex into the aggregate space created by all the micro bubbles. So, even though concrete looks like a rock-solid enemy of the big toe, it is actually an elastic substance that can flex when climatic conditions require it. Cool, right? How on Earth did anyone figure this out?
In 1938 New York (a place whose harsh winter conditions had been beating up road surfaces since the appearance of the automobile), it was noticed that several stretches of roadway were weathering the extreme freezing conditions with surprising success. What these stretches of highway had in common, it was found, was a particular brand of cement, and it was further discovered that this cement’s manufacturing process used tallow-fired kilns.
Tallow (animal fat) was and is a component of the heat-producing fuel in many manufacturing kilns, but tallow has another interesting characteristic. As was already understood by soap manufacturers––who still use it today for its lathering effect––beef tallow is a natural surfactant; that is, it produces bubbles. As it happened, the accidental introduction of the kiln’s beef tallow residue into the concrete company’s product created in the hardened concrete a finely dispersed bubble structure that inadvertently gave the road surface the needed elasticity to flex with the “freeze-thaw” phenomenon.
Since then, concrete manufacturers have so perfected the Air Entrainment process and so maximized its structural benefits, surfactants are a huge global industry within the construction sector. Concrete and soap. Go figure.
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